So far I've just eye-balled them, and most of the time it works.  Does this mean that most gas blocks (I always use low-pros, adjustable ones, etc. - not FSB ones) have larger holes that go over the small hole in the barrel, and therefore you can be not 100% perfect lined up?

Correct.

I've got a jig that lines up the holes for me.  Makes fitting a gas block pretty easy.



I bought it from a member here.

Thanks, guys - sure would suck to go to the range, not have the AR work right due to misaligned gas block, and then try to take it apart and fix it while there - LOL.  I don't think the RSOs would like it much.

I just get it close and if it needs adjustment I will adjust it at the range......private range no RSO's

If your upper receiver is a flat top variety, it make the alignment easier.  Use a set of 1-2-3 blocks and your wife's granite kitchen counter top, place the upper upside-side down, with the receiver on one block and the gas block on another.  You are wanting to get the rail sections parallel to each other.  Once they are flat, tighten your set screws on the gas block, then check after properly torqued.  This measure helps to eliminate any tilting at the front sight in relationship to the rear sight.